During the last decades, the Internet has become ubiquitous by leveraging the World Wide Web, invented by Tim Berners-Lee in 1989, which is nowadays used as a main source of information all over the world. Typically, users access a web server containing the documents they wish to read by means of a service client. The protocol used for accessing the web server is mostly HTTP (hypertext transfer protocol) and the documents are often stored in an html-format. HTML is a format for representing information in a structure that allows for establishing links between the individual documents so that all documents together form a hypertext document structure.
The Internet may also be understood as a network structure including many local area networks (LAN). To provide security measurements so that no data leaves the local area network, firewalls are configured that implement security strategies which guarantee that a security policy with regard to the incoming and outgoing data traffic is complied with.
Frequently, when a user accesses a web server which is beyond the local area network a proxy server is used which stores the contents of web sites that have been visited recently. Thus, the actual web server does not have to be accessed several times, but only a proxy server needs to be accessed. However, the web sites stored on the proxy server may be out-dated after some time, so that the information received loses its value for the user. A proxy device (running either on dedicated hardware or as software on a general-purpose machine) may act as a firewall by responding to input packets (connection requests, for example) in the manner of an application, whilst blocking other packets.
The user of proxies makes it more difficult to tamper with an internal system from the external network, and any misuse of one internal system would not necessarily cause a security breach exploitable from outside the firewall (as long as the application proxy remains intact and properly configured).
A web service is a software application that is identifiable by means of a uniform resource locator and whose interfaces are defined and described as XML-artefacts. A web service supports the direct interaction with other software agents by using XML-based message by the exchange of internet-based protocols.
Businesses now expect their Internet service providers and internal IT departments to provide clear service level guarantees on the availability and response time of the services they provide, along with notifications and resolutions of outages and slowdowns. The term “quality-of-service” (QoS) refers to providing a user who wishes to make use of an IT-infrastructure and services available on that IT-infrastructure with a guarantee concerning the service provided. In a contract which is usually referred to as a “service level agreement” (SLA), the service provider and the service customer agree upon failure times, response times, etc. during the runtime of the SLA. An SLA which allows a service provider to deliver its services with relatively long response times may be less expensive for a customer than an SLA guaranteeing short response times.